Dielectric materials in semiconductor devices in integrated circuits appear as gate dielectrics in dynamic random access memory (DRAM), field effect transistors (FET) and capacitor dielectrics. The dimensions of these dielectrics are related directly to the performance of the semiconductor devices. To achieve faster responses and more complex functionality, integrated circuits are made more compact, e.g., smaller in lateral size and thickness.
The performance of a dynamic random access memory (DRAM) device is related to the charge stored in its capacitor, which change is directly proportional to the area and dielectric constant k of the capacitor, and inversely proportional to the thickness of the capacitor. As the capacitor size decrease, a high-k dielectric is needed in the capacitor to maintain an adequate capacitance charge in a high performance DRAM device.
The operating speed of a semiconductor device is directly proportional to the response of a gate dielectric in a field effect transistor (FET) after a voltage is applied. The response of a gate dielectric is directly proportional to the dielectric constant k and inversely proportional to the dielectric material thickness. Thus, the need for a thin and high-k dielectric is also highly desirable for a gate dielectric.
Silicon dioxide (SiO2) is the conventional material used for gate dielectrics, having a dielectric constant of about 4. As device dimensions are scaled down, the thickness of the silicon dioxide gate dielectric reaches its tunneling limit of between about 1.5 nm to 2 nm. Silicon dioxide films of less than 1.5 nm have a high leakage from direct tunneling currents and generally cannot be used as a gate dielectric in FET devices because of excessive power consumption. Other fabrication and reliability concerns of very thin silicon dioxide films include boron penetration and charge injection damage.
For MOS technology, thermally grown SiO2 on silicon provides the dominant gate structure. The Si/SiO2 interface has excellent properties, including low interface and bulk trapping, thermal stability, high breakdown, etc. With each successive generation of microelectronics technology, however, the thickness of the SiO2 is scaled down. As the gate oxide thickness is scaled below 1.5 nm, problems arise, such as excessive power consumption due to current leakage from direct tunneling, boron dopant penetration, reliability concerns, etc. Because of these problems, SiO2 will likely be replaced by a higher dielectric constant (k) material. A higher k material which will allow the use of a greater thickness to achieve the same capacitance. Requirements for this high-k material include lower leakage, low interface traps, low trapped charge, good reliability, good thermal stability, conformal deposition, etc.
So far, a suitable replacement for SiO2 has not been found. The most promising candidate materials are metal oxides, such as HfO2 and HfxAlyOz. Because of the requirements for conformality and thickness control, atomic layer deposition (ALD) has emerged as one of the most promising deposition techniques. In this technique, dielectric material is built up layer-by-layer in a self-limiting fashion, i.e., the deposition phenomenon where only one monolayer of a chemical species will adsorb onto a given surface.
In ALD, the precursor vapors are injected into the process chamber in alternating sequences: precursor, purge gas, reactant, purge gas. The precursor adsorbs onto the substrate and subsequently reacts with the reactant. There are various modifications of the ALD processes, however, basic ALD processes all contain two distinct properties: alternating injection of precursors and the saturation of the precursor adsorption. By alternating precursors and reactants in the vapor stream, separated by a purge gas, the possibility of gas phase reaction is minimized, allowing a wide range of possible precursors. Also, because of the self-limiting nature of the chemisorption mechanism, the deposited film is extremely uniform, because once the surface is saturated, the additional precursors and reactants will not further adsorb or react and will be exhausted away.
The precursor requirements of ALD are different from those of CVD because of the different deposition mechanisms. ALD precursors must exhibit a temperature range between condensation and decomposition and have a self-limiting effect, so that only a monolayer of precursor is adsorbed on the substrate. Precursors designed for ALD use must readily adsorb at bonding sites on the deposited surface in a self-limiting mode. Once adsorbed, the precursors must react with the reactants to form the desired film. In CVD, the precursors and the reactants arrive at the substrate together and the film is deposited continuously from the reaction of the precursors with the reactants. The deposition rate in CVD process is proportional to the precursor and reactant flow rate and to the substrate temperature. In CVD, the precursor and the reactant must react at the deposited surface simultaneously to form the desired film. Thus many useful CVD precursors are not viable as ALD precursors and vice versa. It is not trivial or obvious to select a precursor for the ALD method.
Currently, the leading ALD precursors for depositing metal oxides are metal organics, metal halides, metal amides, and anhydrous metal nitrates. These precursors all have serious drawbacks. Metal organics have the potential to introduce organic contamination into the IC structure during the fabrication process, leading to degraded IC performance.
HfO2 and ZrO2 films deposited using a metal halide precursor, such as [M+]Cl4, demonstrate good insulating properties, including high dielectric constant and low leakage, however, they do not provide smooth deposition directly on H-terminated silicon and require several “incubation” cycles, which may result in roughening. For uniform initiation, they require several monolayers of SiO2 or Si3N4. Because even a very thin low-k layer can negate most of the benefits of an overlying high-k layer, the presence of several monolayers of SiO2 makes it difficult to achieve EOT<1.0 nm using this type of precursor. Deposition directly on hydrogen-terminated silicon is essential. This initiation problem must be solved before metal-chloride precursors can be used in production.
The nitrate (NO3) ligand is a powerful oxidizing and nitriding agent, and capable of reacting strongly with many compounds. Gates et al., U.S. Pat. No. 6,203,613, granted Mar. 20, 2001, for Atomic layer deposition with nitrate containing precursors, describes an ALD method using metal nitrate precursors in conjunction with oxidizing, nitriding and reducing co-reactants to deposit oxide, nitride and metal films, respectively.
Ono et al., in U.S. Pat. No. 6,420,279, granted Jul. 16, 2002, for Method of using atomic layer deposition to deposit a high dielectric constant material on a substrate, for example, describes ALD deposition of zirconium oxide using zirconium nitrate precursor, together with an oxidizing agent, such as water or methanol; and the ALD deposition of hafnium oxide using a hafnium nitrate precursor together with an oxidizing agent, such as water or methanol, which patent is incorporated herein by reference.
A metal nitrate precursor, Hf(NO3)4 has recently been demonstrated as a viable ALD precursor, in the first-above-identified related Application and in Conley et al., Atomic layer deposition of hafnium oxide using anhydrous hafnium nitrate, Electrochemical and Solid-State Letters, 5 (5), C57-59 (2002). The primary benefit of Hf(NO3)4 is that it allows deposition initiation directly on H-terminated silicon, resulting in a uniform thin layer, without the requirement of an initial layer of low-k material. However, as previously noted, HfO2 films deposited via ALD of Hf(NO3)4 have a dielectric constant that is lower than expected, possibly due to the low density of the films (˜8.5 g/cm3 vs. a bulk value of 9.7 g/cm3), even after a post deposition anneal of up to 850° C. The low film density may be a result of the low deposition temperature (˜170° C.) required for use of this precursor. Before metal-nitrate precursors can find widespread use, the “bulk” dielectric properties of the resulting films must be improved.
In addition, films produced by all of the above-described methods suffer from poor channel mobility when integrated into an MOS device structure.
Yet another problem with traditional method of ALD is the presence of silicon oxide at the wafer interface. This layer forms because, in addition to the requirement for a metal precursor, an oxidizing precursor is also required, which promotes formation of silicon oxide. The oxidizing precursor produces an unwanted SiO2 layer between the wafer surface and the metal oxide film.
The use of water vapor introduces a number of problems: The introduction of the necessary quantity of water vapor is difficult to control, and typically, an abundance of water vapor is introduced into the chamber. Water is a polar molecule, and adsorbs efficiently onto surfaces, i.e., the chamber walls. Desorption of water vapor, e.g., via vacuum pumping, adds considerable time to the overall process time, reducing the throughput.
The second through fourth above-identified related patents and patent applications describe methods to improve the quality of the ALD metal-oxide films in which a combination of precursors is used. The use of a combination of precursors provides the advantages of each precursor while minimizing the disadvantages. The “dual-precursor” methods described in the related applications allows for more efficient ALD of HfO2, while negating the need for a separate oxidizing source, such as H2O. However, although the films received a post deposition densification anneal, it was found that the density was still only ˜8.5 g/cm3, much lower than the reported bulk value.